1. Field of the Invention
The present invention relates to a discharge lamp lighting device for lighting a high-intensity discharge lamp used in a projection-type video display apparatus, and a projection-type video display apparatus that uses a discharge lamp lighting device.
2. Description of Related Art
Examples of projection-type video display apparatuses (hereinafter, also may simply be referred to as “projectors”) include liquid crystal projectors that yield a projected video image with transmitted light through a liquid crystal display element, and digital light processing (DLP) (registered trademark) projectors that yield a projected video image with a light reflected by a reflection-type mirror device element such as a digital micro-mirror device (DMD) element. As the white light source used for projectors, high-intensity discharge lamps such as a high-pressure mercury lamp and a xenon lamp are often used.
When discharge lamps are driven by a DC power source, electrons collide with the anode during arc discharge and the temperature of the anode becomes higher than that of the cathode, resulting in a reduced lamp life. For this reason, discharge lamps are driven by alternating the anode and the cathode periodically by applying an AC current at a predetermined frequency to the electrode pair.
As a method for preventing the occurrence of a so-called flicker by stabilizing the position of an arc bright spot in a discharge lamp, JP 2006-059790A discloses a technique by which the discharge lamp is lighted by supplying an AC current with a steady-state lighting frequency f0, and inserting an AC current with a low frequency f1 that is lower than the steady-state lighting frequency f0.
In the case of driving a discharge lamp with an AC current, the current momentarily becomes 0 upon reversal of the current directions, and therefore the lamp brightness is reduced, resulting in a momentary reduction of the projection light amount. In the case of the method disclosed in JP 2006-059790A, the momentary reduction of the projection light amount occurs with different timing between a lamp current with the steady-state lighting frequency f0 and a lamp current with the low frequency f1. That is, even at the timing when the projection light amount is reduced momentarily upon polarity reversal of the lamp current of the steady-state lighting frequency f0, the reduction of the projection light amount does not occur during a period without a polarity reversal in the lamp current at the low frequency f1.
FIG. 6 (a) schematically shows a waveform example of a lamp current in the method disclosed in JP 2006-059790A. FIG. 6 (b) shows an example of the changes in the projection light amount due to the current waveform. At the time of polarity reversal of the current with the steady-state lighting frequency f0, a brightness reduction waveform 10 as shown in FIG. 6 (b) is generated, and the projection light amount is momentarily reduced. An enlarged view of the brightness reduction waveform 10 is shown in FIG. 7. On the other hand, the reduction of the projection light amount does not occur during a period without a polarity reversal in the lamp current at the low frequency f1. The locations indicated by oval FIG. 20 in FIG. 6 (b) correspond to timings at which the reduction of brightness does not occur in the case of the low frequency f1 even if the reduction of brightness occurs in the case of the steady-state lighting frequency f0.
According to a drive waveform without a period of a lamp current of the low frequency f1, the reduction of the projection light amount occurs regularly at intervals of polarity reversal of the current with the steady-state lighting frequency f0, which is a comparatively high frequency (at least 60 Hz). Therefore, it is unlikely that flicker resulting from this brightness reduction waveform 10 is perceived.
In contrast, with a drive waveform having a period of the low frequency f1, a period without a reduction of the projection light amount (a period without a polarity reversal in the current at the low frequency f1, which corresponds to the locations indicated by the oval FIGS. 20 in FIG. 6 (b)), is inserted during repeated reductions of the projection light amount occurring at intervals of polarity reversal of the steady-state lighting frequency f0. This may be perceived as flicker.
When a lamp current in which an AC current with the low frequency f1 is inserted into an AC current with the steady-state lighting frequency 10 is used for a one-chip DLP projector, the following problem occurs. FIG. 8 shows an example of a typical four-color (R, G, B, W) color wheel used for a one-chip DLP projector. FIG. 9 shows an example of the state of projection light and so forth for the above-described case. FIG. 9 (a) shows the lamp current, FIG. 9 (b) shows on/off timing in a single pixel of the video display element (DMD), and FIG. 9 (c) shows the changes in the projection light amount.
In the case of a one-chip DLP projector, usually the polarity reversal of the steady-state lighting frequency f0 is performed at boundaries between segments of the color wheel. The purpose of this is to minimize the reduction of the projection light amount due to the polarity reversal and the deterioration of the video image quality caused by overshoot, ringing, and the like. As shown in FIG. 9 (a), in the lamp current with the steady-state lighting frequency f0, the polarity reversal is performed at boundaries between the color segments (R, G, B, W). Further as shown in FIG. 9 (b), the pixel is switched between on and off at a timing corresponding to the middle of the width of the lamp current that corresponds to each of the color segments.
Ordinarily, a color boundary rarely is contained in a period where each pixel is turned on. However, in order to enable detailed video expression, there may be cases where boundary-on waveforms 13 and 14 containing a color boundary are used. Consequently, boundary light amount waveforms 15 and 21 are contained in the projection light, as shown in FIG. 9 (c). Enlarged views of these boundary light amount waveforms 15 and 21 are as shown in FIGS. 10 and 11. The boundary light amount waveform 15 in FIG. 10A contains a brightness reduction waveform in which the projection light amount is reduced due to the influence of the polarity reversal of the steady-state lighting frequency f0. On the other hand, there is no reduction of the projection light amount in the boundary light amount waveform 21 in FIG. 11 since this waveform is in the period without a polarity reversal at the low frequency f1.
As described above, in the example of the one-chip DLP projector in FIG. 9 as well, a period without a reduction of the projection light amount is inserted during repeated reductions of the projection light amount occurring at intervals of the polarity reversal at the steady-state lighting frequency f0, and therefore this may be perceived as flicker. Moreover, when the number of on-periods is small as shown in FIG. 9 (b) and so the total brightness is low, the ratio of (reduction of projection light amount indicated by the boundary light amount waveform 15)/(total brightness) becomes high, further increasing the possibility of being perceived as flicker.